1. Field of the Invention
This invention relates to lasers, in general and to improved reflective mirrors for use with lasers, in particular.
2. Prior Art
Lasers are devices which are known in the art as devices for generating coherent light usually in the form of a relatively narrow, parallel beam of monochromatic light. Lasers and laser beams can exhibit high power characteristics.
Lasers designed for high output power are usually used in applications which require that this power (beam) can be focused to a spot size which is so small that the dimensions of the spot are limited only by the well-known laws of optical diffraction. However, in designing a high power laser it is often desirable to use large diameter optical and mechanical components. Unfortunately in order to maintain a diffraction limited laser output beam to be able to be focused to a diffraction limited spot, the laser had to be oscillating in the TEM.sub.00 mode (as is well known to those skilled in the art of laser). However, in order to obtain TEM.sub.00 mode oscillation from a laser, some method of discriminating against higher order laser modes must be employed. The most common method used is to design the laser with a low Fresnel number cavity. The Fresnel number of a laser is given by the following formula:
N=a.sup.2 /.lambda.d
Where:
N=Fresnel number
.lambda.=Laser wavelength
d=Distance between the laser mirrors
a=Aperture radius.
Particularly "a" is the radius of the internal aperture which determines the diameter of the laser beam at which diffraction losses occur. Normally "a" would be the inside radius of the laser tube but with a multi-cavity mirror, the limiting aperture is the radius of the individual cavities as described hereinafter.
In lasers, such as CO.sub.2 lasers, which are designed for high output powers it has been found experimentally that a Fresnel number of less than one usually results in TEM.sub.00 mode oscillation while numbers greater than 3 usually result in higher order mode oscillation. For numbers between 1 and 3, the mode of oscillation depends on many factors including the curvature of the mirrors, the gain of the laser and the like. Inasmuch as the number is proportional to the radius squared, it can be seen that there is a limit to the maximum radius which is allowed for a given mirror separation and wavelength. However, frequently it is found that this maximum radius is smaller than the optimum radius for generating the highest laser powers.
One method of obtaining single mode output from lasers with large Fresnel numbers is to use a laser mirror design commonly referred to as an unstable resonator. However, this approach has several undesirable features which include for example, extremely critical mirror alignment, an annular output beam, and a loss in laser efficiency.